The optical characteristics of a material are important material properties, and can be used, for instance, to assign optical signatures to well-known objects or classes of objects, and to identify such objects or classes of objects remotely. For an opaque object, i.e., one having zero transmittance, the object's directional emittance can be characterized if one knows the object's directional hemispherical reflectance as a function of object temperature and angle of incidence. Many systems for determining reflectance are known, prominent among which are integrating spheres, which for decades have been used to measure the reflectance of diffusely reflecting materials in the UV, visible, and near IR. Unfortunately, there are no generally agreed upon reflectance standards beyond 2.5 micrometers in the infrared. Consequently the reflective properties of materials in the infrared are not well known, and there is a need for integrating sphere systems which can measure the infrared diffuse reflectivity of materials with efficiency, convenience, and reliability. Infrared measurements are complicated by air having several constituents (e.g. water and carbon dioxide), that absorb at infrared frequencies, which can distort or otherwise make less precise such measurements of diffuse reflectance if the measurements are made in an air atmosphere with a single beam spectrophotometer. Unfortunately, were one to contain any of the present integrating sphere systems in a chamber containing an artificial, non-absorptive atmosphere, one could examine the angular dependence an object's diffuse reflectance only by venting the atmosphere after each test at each angle of incidence, repositioning the object to change the angle of incidence, and recharging the system's artificial atmosphere. This repeated venting and recharging is most inefficient, inconvenient, and uneconomical.